1. Field of the Invention
This invention relates to a device for efficiently heating a fluid material such as a high polymer material by microwave energy in the course of conveying the material under pressure, as, for example, in a rubber production process. The invention also relates to a device for heat crosslinking a high polymer material in a molding machine.
2. Description of the Prior Art
The perspective view of FIG. 2(a) and the cross-sectional view of FIG. 2(b) illustrate the prior art method for heating a high polymer or other fluid using microwaves. In the illustrated method, microwave energy is guided in the direction of the arrow a.sub.1 within a waveguide 1 while a fluid L to be heated is passed through tube 2 made of a dielectric material, and the fluid L is heated in accordance with the principle of dielectric heat generation.
Suitable materials for the dielectric tube 2 used in this method include glass, ceramic and synthetic resins. However, when, as discussed later, an attempt is made to improve the heating efficiency by reducing the wall-thickness of the dielectric tube 2 made of such a low-strength material, the maximum pressure which can be used for pressurized conveyance of the fluid is no more than around 10 Kg/cm.sup.2.
In an application of microwave heating to a process involving pressurized conveyance of a plasticized rubber of synthetic resin, however, since the viscosity of the plasticized material is high, it is impossible to realize smooth conveyance of the material unless there is used a high conveyance pressure in the range of 50 Kg/cm.sup.2 to 2000 Kg/cm.sup.2.
In the method shown in FIG. 2, a dielectric tube 2 capable of withstanding a pressure of 2000 kg/cm.sup.2, for example, would have to have a wall thickness of 40 mm or more if made from quartz glass and of 30 mm or more if made from ceramic alumina. While such a tube is both difficult and expensive to fabricate, these are not the major disadvantages encountered when attempting to increase the tube's ability to withstand pressure merely by increasing its wall thickness. An even greater problem is that discussed below.
Consider the case where ceramic alumina is used as the material for the dielectric tube 2. In this case, the specific inductive capacity of the ceramic alumina would, at 9.8, be larger than the specific inductive capacities of about 2.5 to 4.5 of the rubber or synthetic resin to be heated. As a result the fluid L to be heated would be surrounded by a material of a thickness greater than one-half the wavelength of the microwaves and having a specific inductive capacity which is 2 to 4 times greater than its own specific inductive capacity. Under such circumstances, the microwave field would pass straight through the ceramic alumina with the high specific inductive capacity and the fluid L to be heated would be left unaffected.
What this means then is that in carrying out heating of a fluid using the arrangement illustrated in FIG. 2, it is possible to realize efficient microwave heating of the fluid L within the tube only if the wall thickness of the tube is made very small in comparison with the wavelength of the microwaves or, alternatively, the specific inductive capacity of the dielectric tube is not larger than that of the fluid L which is to be heated. Since the upper temperature limit in the heating of a rubber or synthetic resin is in the neighborhood of 200.degree. C., it would appear feasible to use a tube made of tetrafluoroethylene, which has a specific inductive capacity of 2.1, a dielectric dissipation factor of 0.0005 and heat resistance up to 265.degree. C. However, as tetrafluoroethylene resin easily deforms even under a low pressure, it would not be capable of withstanding pressures up to 2000 kg/cm.sup.2. It is thus not suitable for use in heating a high-pressure fluid in accordance with the method of FIG. 2.
The prior art has further drawbacks in connection with the application of the microwave heating device to a line for extrusion or injection molding of rubber and the like. As shown in FIG. 5, the prior art line for extrusion molding of rubber and the like comprises an extrusion molding machine 101, a microwave heating device 102 and a secondary heating device 103, provided separately as independent units. Material charged into a hopper 104 is conveyed within a cylinder 105 in the direction of a forming die 107 by a screw 106. The material arriving at the forming die 107 is pushed out through the opening therein in the form of a bar 108 which is supplied to a microwave heating device 102. After being heated to crosslinking temperature by microwave heating, the material is sent to a secondary heating device where it undergoes crosslinking or polymerization reaction.
There are two reasons for providing the extrusion molding machine and the microwave heating device as separate units in this fashion.
(1) The screw 106 conveys the high polymer material under a pressure of 100-500 kg/cm.sup.2 and the temperature within the extruder head rises 80.degree.-160.degree. C. Under such conditions it is difficult to build the microwave heating device into the extruder head.
(2) Generally speaking, high polymer materials exhibit poor microwave absorption so that it is difficult using microwaves to heat a small volume of high polymer such as that present within the extruder head. Therefore, as shown in the figure, it has been the practice to provide a separate microwave heating device 102 so as to be able to employ a large microwave irradiation chamber that can accommodate a large volume of high polymer material, in this way enhancing microwave absorption.
However, this type of line arrangement is disadvantageous in that the equipment becomes complicated, bulky and difficult to repair and maintain.
Because of these problems, various proposals have been made regarding ways for raising the high polymer material to the required temperature before extrusion. Typical of these is the arrangement shown in FIG. 6, in which the high polymer material is forced into a heating cylinder 109 from the head of the extrusion molding machine, a rotor 110 housed within the heating cylinder 109 is rotated at the high speed by a motor M and the high polymer material 111 present between the heating cylinder 109 and the rotor 110 is heated by frictional and shearing forces prior to extrusion through a die 112.
While the illustrated arrangement enables heating through control of the rotational speed of the rotor 110, it frequently leads to burning since, there being no way to limit the heating zone to a narrow region immediately preceding the die, the high polymer material is apt to undergo crosslinking reaction before reaching the die. It is thus impossible to readily bring the high polymer material to a high temperature in the region of the die. Moreover, since the high polymer material is heated mechanically by frictional and shearing forces exerted thereon in the course of its conveyance under pressure, there is a risk of the high polymer material being converted into a low polymer material by shearing. What is more, temperature control is difficult.
When a high polymer material is subjected to crosslinking reaction, the viscosity of the high polymer varies with time, as shown in FIG. 7. More specifically, as shown in the drawing, as the temperature rises the viscosity of the high polymer first decreases, and then, with further heating, increases because of crosslinking. Moreover, the time required for completion of crosslinking varies depending on the heating temperature. In FIG. 7, the characteristics of the change in viscosity for low and high temperature heating are represented by the broken and solid line curves, respectively. When the temperature is too low, the high polymer molded by the die is apt to deform under its own weight before curing is complete. On the other hand, if the temperature is too high, crosslinking is completed before the high polymer material passes through the die, which hinders smooth die molding operation and leads to deterioration of the high polymer material.
The best mode of heating is thus one carried out in a very narrow zone immediately before the die, and one which brings the high polymer material to the desired temperature in minimum time.